1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material comprising a thermosensitive coloring layer and a protective layer successively formed on a support, which protective layer comprises a resin and a filler comprising aluminum hydroxide or/and aluminum oxide particles, having a volume mean diameter of 1 .mu.m or less, with the amount of the particles with a diameter of 4 .mu.m or less being 90 vol. % or more in the entire particles.
2. Discussion of Background
Conventionally, there is known a thermosensitive recording material, in which a thermosensitive coloring layer mainly comprising a thermosensitive coloring composition is provided on a support such as a sheet of paper or synthetic paper, or a plastic film. In such a recording material colored images are obtained by application of heat to the recording material using a thermal printer provided with a thermal head.
This type of thermosensitive recording material is widely used, not only as a recording material for copying books and documents, but also as a recording material for use with computers, facsimile apparatus, teleprinter exchanger, medical instruments, measuring instruments, ticket vendors, and label printing machines for the POS system in supermarkets and department stores, because of the following advantages over other conventional recording materials:
(1) images can be speedily recorded by using a comparatively simple device without complicated steps such as development and image fixing;
(2) images can be recorded without generating noise; and
(3) the manufacturing cost is low.
When such a thermosensitive recording material is used as a thermosensitive adhesive label for the POS system of supermarkets and department stores, characters and bar codes representing the product name and price are thermally recorded on the thermosensitive adhesive label using a thermal printer provided with a thermal head, and the thermally printed label is applied to a variety of commercial products, with the backing sheet of the label removed therefrom. Thus, customers can identify the name and price of the product and other information concerning the product by the label. Furthermore, the bar codes thermally printed on the thermosensitive adhesive label can be read by an automatic bar code reader at the point of registration and the bar code information is processed by computer, which promotes the sales management and stock control, and thus facilitates the reorder of products.
When such thermosensitive recording adhesive labels are employed, particularly in food packaging, it is required that thermally printed images on the labels be prevented from becoming blurred or fading away even if they are brought into contact with oils contained in foods and plasticizers contained in a plastic wrapping film.
In order to satisfy the above requirements, various proposals have been made. For example, in Japanese Patent Publication No. 58-39078, a protective layer comprising a resin such as polyvinyl alcohol and a filler is formed on a thermosensitive coloring layer, thereby preventing the thermosensitive coloring layer from coming into contact with the oil and the plasticizer. However, when such a protective layer comprising the resin as the main component is overlaid on the thermosensitive coloring layer, the resin is fused and adheres to the surface of a thermal head while the thermal recording is carried out under application of thermal energy from the thermal head. Thus, a so-called "head-dust" gradually accumulates on the thermal head and the sticking of the recording material to the thermal head occurs.
In order to solve the above problems, various organic and inorganic fillers to be employed in the protective layer have been studied. For example, Japanese Laid-Open Patent Application No. 58-208091 discloses a protective layer comprising as a filler silicon dioxide; and Japanese Laid-Open Patent Application No. 62-59081, aluminum hydroxide.
When the protective layers comprising the above-mentioned fillers are employed, however, all the requirements cannot be satisfied in practice. More specifically, in the case where the head-dust problem and the sticking problem are solved in some degree, the thermal head is worn out. When the thermal head is prevented from wearing, the head-dust problem occurs. The conventional fillers cannot satisfactorily serve to solve both the head-dust problem and the head-wearing problem at the same time.